Field of the Invention
The present disclosure relates to solar cells and the fabrication of solar cells, and more particularly the design and specification of a multijunction solar cell using electrically coupled but spatially separated semiconductor bodies based on III-V semiconductor compounds.
Description of the Related Art
Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology not only for use in space but also for terrestrial solar power applications. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to properly specify and manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 27% under one sun, air mass 0 (AM0) illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions.
In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads become more sophisticated, and applications anticipated for five, ten, twenty or more years, the power-to-weight ratio and lifetime efficiency of a solar cell becomes increasingly more important, and there is increasing interest not only the amount of power provided at initial deployment, but over the entire service life of the satellite system, or in terms of a design specification, the amount of power provided at the “end of life” (EOL).
The efficiency of energy conversion, which converts solar energy (or photons) to electrical energy, depends on various factors such as the design of solar cell structures, the choice of semiconductor materials, and the thickness of each subcell. In short, the energy conversion efficiency for each solar cell is dependent on the optimum utilization of the available sunlight across the solar spectrum as well as the “age” of the solar cell, i.e. the length of time it has been deployed and subject to degradation associated with the temperature and radiation in the deployed space environment. As such, the characteristic of sunlight absorption in semiconductor material, also known as photovoltaic properties, is critical to determine the most efficient semiconductor to achieve the optimum energy conversion to meet customer requirements of intended orbit and lifetime.
Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures or stacked sequence of solar subcells, each subcell formed with appropriate semiconductor layers and including a p-n photoactive junction. Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current. After the sunlight impinges on the front of the solar cell, and photons pass through the subcells, the photons in a wavelength band that are not absorbed and converted to electrical energy in the region of one subcell propagate to the next subcell, where such photons are intended to be captured and converted to electrical energy, assuming the downstream subcell is designed for the photon's particular wavelength or energy band.
The individual solar cells or wafers are then disposed in horizontal arrays or panels, with the individual solar cells connected together in an electrical series and/or parallel circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
The electrical characteristics of a solar cell, such as the short circuit current density (Jsc), the open circuit voltage (Voc), and the fill factor, are affected by such factors as the number of subcells, the thickness of each subcell, the composition and doping of each active layer in a subcell, and the consequential band structure, electron energy levels, conduction, and absorption of each subcell, as well as its exposure to radiation in the ambient environment over time. The overall power output and conversion efficiency of the solar cell are thereby affected in different and often unpredictable ways. Such factors also vary over time (i.e. during the operational life of the system).
Accordingly, it is evident that the consideration of any one design parameter or variable, such as the amount of a particular constituent element in a layer, or the band gap of that layer, affects each of the electrical characteristics in a different way, sometimes in opposite directions, and such changes does not predictably lead to an increase in power output or solar cell efficiency. Stated another way, focus on any one such parameter in the design of a multijunction solar cell is not a viable calculus since each variable standing alone is NOT a simple “result effective” variable that can be simplistically adjusted by those skilled in the art confronted with complex design specifications and practical operational considerations in order to achieve greater power output or a related design objective of the solar cell.
Another parameter of consideration taught by the present disclosure is the difference between the band gap and the open circuit voltage, or (Eg/q−Voc), of a particular active layer, and such parameter may vary depending on subcell layer thicknesses, doping, the composition of adjacent layers (such as tunnel diodes), and even the specific wafer being examined from a set of wafers processed on a single supporting platter in a reactor run.
One of the important mechanical or structural considerations in the choice of semiconductor layers for a solar cell is the desirability of the adjacent layers of semiconductor materials in the solar cell, i.e. each layer of crystalline semiconductor material that is deposited and grown to form a solar subcell, have similar crystal lattice constants or parameters. Although such conventional wisdom suggests that all subcells be lattice matched to achieve the most power output, the present application departs from that conventional wisdom in that at least one subcell is not lattice matched with the other matched subcells.